1 Technical Field
The present invention relates to double-stator rotating electric machines which include a rotor, an outer stator disposed radially outside the rotor, and an inner stator disposed radially inside the rotor.
2 Description of Related Art
Japanese Patent Application Publication No. JP2007282331A discloses a double-stator electric motor. The motor includes a rotor, an outer stator disposed radially outside the rotor, and an inner stator disposed radially inside the rotor. The rotor includes an annular rotor core, a plurality of outer permanent magnets and a plurality of inner permanent magnets. The rotor core has a plurality of fitting holes and a plurality of protrusions. The fitting holes are formed in close vicinity to a radially outer surface of the rotor core and spaced from one another in a circumferential direction of the rotor core. The protrusions are formed on a radially inner surface of the rotor core and spaced from one another in the circumferential direction. Each of the outer permanent magnets is fitted in one of the fitting holes of the rotor core. Each of the inner permanent magnets is mounted on the radially inner surface of the rotor core so as to be circumferentially interposed between an adjacent pair of the protrusions of the rotor core.
Consequently, with the outer permanent magnets fitted in the corresponding fitting holes of the rotor core, it is possible to reliably prevent the outer permanent magnets from being detached from the rotor core during high-speed rotation of the rotor. Moreover, with the protrusions of the rotor core, it is possible to reliably prevent deviation in circumferential position of the inner permanent magnets. Furthermore, with the inner permanent magnets mounted on the radially inner surface of the rotor core, it is possible to reduce the radial gap between the inner permanent magnets and the inner stator, thereby increasing the output torque of the motor.
However, during operation of the motor, both the outer and inner stators generate heat. In particular, the inner stator is entirely surrounded by the rotor and a motor housing. Consequently, it is difficult to supply coolant to the inner stator; thus, it is difficult to dissipate the heat generated by the inner stator.
To solve the above problem, one may consider increasing the cross-sectional area of a coil of the inner stator and thereby reducing the heat generated by the coil during operation of the motor. However, in this case, the size of the entire motor would be increased accordingly. In other words, the size of the motor would become large for the output torque of the motor.